Wireless communication systems are required to transmit ever-increasing amounts of data, in support of expanded subscriber services, such as messaging, e-mail, music and video streaming, and the like. Transmitting a higher volume of data over a given channel requires transmission at a higher data rate.
One known technique to improve data transmission rates in wireless communications is the use of multiple input, multiple output (MIMO) technology, wherein signals are transmitted from multiple transmit antennas and may be received by multiple receiver antennas. Using advanced coding and modulation schemes, two or more streams of data may be transmitted simultaneously to a receiver, increasing the data rate.
Maintaining high data rates in MIMO systems requires fast link adaptation. That is, the transmitter must constantly alter its selection of transmission parameters, such as the transmission frequency band and transmission antenna configuration, based on the current characteristics of the channel, which can change rapidly. In a Frequency Division Duplex (FDD) system, the instantaneous downlink channel conditions are not available at the base station; they must be determined by a receiver and communicated to the base station. In Wideband CDMA (WCDMA) and Long Term Extension (LTE), the instantaneous downlink channel conditions are communicated to the base station through a Channel Quality Indicator (CQI).
Estimating the CQI is a delicate task. Ideally, the channel quality should be estimated not only once, but for different potential frequency bands (if applicable) and for different potential Virtual Antenna Combinations (VAC) (if applicable). A virtual antenna consists of one or more antenna elements. These estimates are preferably performed for all potential combinations and at every Transmission Time Interval (TTI), in order to achieve maximum performance by selecting the optimal transmission parameters. This is necessary in order to determine the rank of the channel and which antennas to use for transmission, and also the optimal transmission frequencies. Furthermore, each estimate should be performed using all received reference (pilot) symbols, to ensure the highest quality channel estimates.
However, given the computational resources and power consumption constraints of mobile User Equipment (UE), it is virtually impossible to estimate the channel quality for all potential VACs and potential frequency bands for each TTI and at all reference symbol positions.